Pulsed radio signaling



4 Sheets-Sheet l uw n NN IPL www Oct. 14, 1952 E. s. PURlNGToN FULSEID RADIO SIGNALING Original Filed May 18, 1944 NW .um mm F H wm fw\\ \\NN \N.\\ QMS QM c ww) N S INVVENTOR. Elli/)011 Parilzgozz BY A ATTORNEY.

Oct. 14, 1952 E. s. PURINGTON PULSED RADIO SIGNALING 4 Sheets-Sheet 2 INVENToR.

IIIIII Original Filed May 18, 1944 4 www@ m w m u M w n JU V u NN NRI. OIII ANN U @uml m .n w m .M A

Oct. 14, 1952 E. s. PURINGTON PULSED RADIO SIGNALING 4 Sheets-Sheet 5 Original Filed May 18, 1944 ATTORNEY Oct. 14, 1952 E. s. PURINGTON 2,614,210

PULsED RADIo SIGNALING original Filed May 1e, 1944 4 sheets-sheet 4 E21' ll- IT Z ATTRNEY Patented Oct. 14, 1952 UNITED STATES ATENT OFFICE PULSED RADIO SIGNALING Ellison S. Purngton, Gloucester, Mass., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Cl. Z50- 10) 6 Claims. l

This application is a division of my copending U. S. application Serial No. 536,104, led May 18, 1944, entitled Radio Control System, which matured into Patent No. 2,465,925, granted March 29, 1949, assigned to the same assignee as the instant application.

This invention relates to a radio control system suitable for use in ultra-high frequency channels and has for an obj ect to provide a system of the above type adapted for radio dynamic control or for communication purposes, which is highly selective and which has characteristics such that it is free to a high degree from interference.

Another object is to provide a system of the above type having novel and improved details of construction and features of operation.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

The present system utilizes two channels in the ultra-high frequency range, such as in the range of 400 to 1000 megacycles, and provides for the transmission of short, timed pulses on the two channels which may be varied as to sequence in accordance with the desired control or signal. The system utilizes a pulse transmitter which is capable of transmitting short pulses of comparatively high power.

A radiant energy pulse is sent from the transmitter to the receiver on one channel followed by a radiant energy pulse on the other channel. There is a frequency separation between the two channels on which energies necessary for operation are sent, and there is a time separation between the arrival of the two energies at the receiver. The freedom from interference depends largely upon the amount of frequency separation of the channels and upon the amount of time separation of the energies on the channels. A most important factor is the use of transmitter energies in the form of short dots or pulses with Very high peak radiant power, and With relatively long intervals between the pulses.

By the combination of pulse type transmission, two radiant energy channels and different times of transmission of the energies on the two channels, an extremely high degree of security is provided.

In the present system the pulses may be from 10 to 50 microseconds in length. The time interval between the pulses is several times the length of the pulses themselves, for example, about to 20 times the length of the pulses and successive pairs of pulses are separated by a space having a duration several times the interval between the 2 pulses themselves, for example, 20 to 100 times said time interval.

The control may be responsive to the receipt of a single pair of pulses. Hence the complete transmission may be of extremely short duration, which renders its interception and interference extremely dinicult. However, the transmissions may extend over a plurality of pairs of pulses ii' desired, in which case, accidental interference with one or more of the pairs of pulses would not aiect the control.

While it is known to be physically possible and practicable to produce high peak power on a narrow radio band over a short duration of time, it is not practicable to produce a comparable high peak power over both a Wide radio band and over a long duration of time, as would be necessary for successful interference purposes. Ordinary, non-pulse transmitters would not be capable of interfering successfully with the control because sufcient power would not be available to produce a response at the receiver matching in intensity the pulse from the pulse transmitter. On the other hand, with a pulse transmitter it would be very unlikely that the interfering pulse would coincide in time with a control pulse, or that its rate of recurrence would correspond to that of the control pulse as would be required to produce interference on succeeding pulses. For high speed telegraphic purposes, for example, a single dot or dash might occasionally be obliterated by interference, but for slow speed or radio control purposes, this amount of interference would be entirely negligible.

In accordance with the present invention, a pulse forming circuit is designed to form a succession of pairs of spaced pulses. The pulses of each pair are separated and are used individually to modulate radio transmitting means operating on two diiierent channels, the rst pulse of each pair being connected to modulate the transmitter on one channel and the second pulse of each pair being connected to modulate the transmitter on the other channel. Selective means is provided in the modulating circuit so that the iirst pulse of each pair may be applied to either channel at will and the second pulse may be applied to the other channel. In this way, the pulses are radiated over the two channels -with a frequency sequence determined by the selective means and with a time interval determined by the original pulse forming circuit.

In a specic embodiment of the invention the pulses in the two channels are received by separate receivers tuned to the respective channels and containing the usual detector circuits to make the pulses available in an output circuit. The outputs of the two receivers are connected to a timing comparator which is designed to be responsive only when the pulses are received with a predetermined time spacing.

In one embodiment the timing comparator includes a pulse stretching circuit. which is arranged to stretch the pulse from one of the receivers so as to cause the same to overlap in time the pulse from the second receiver when the rst pulse is received a predetermined time interval before the second pulse. The combined eiect of the two pulses, when they are thus caused to overlap, is utilized to actuate an output relay circuit. When only a single pulse is received or when the pulses are not received in proper time sequence to cause the same to overlap in the output circuit of the comparator no response is produced.

The effect of two overlapping pulses separately applied to the timing comparator is to produce a single pulse of a duration equal to the amount of time of overlap. This can be applied to a relay system operative on a' single pulse, as for example a gaseous type electronic relay, which however requires a reset device such as the use of A. C. plate supply if the control operation is to be repeated. While this is satisfactory for some purposes, it is in general preferable to actuate the control from the combined effect of a considerable number of recurrent pulses giving additional security by electrical tuning to the recurrence rate.

Instead of utilizing two separate receivers the incoming radiant energy may be received in a single receiver tuned to respond to both channels. The receiver will then include suitable pulse separation means to separate the pulses received in the respective channels and to apply them to the timing comparator above outlined.

The invention will be described in greater detail by reference to the accompanying drawings in which:

Figure l is a block diagram indicating the general arrangement of a transmitter and receiver embodying the present invention;

Figure 2 is a schematic diagram of a pulse forming circuit for use in the system of Fig. 1;

Figure 2A is a series of curves illustrating the operation of the pulse forming circuit;

Figure 3 is a schematic diagram of a timing comparator circuit for use in the receiver of Fig. 1; and

Figure 4 is a schematic diagram of a pulse receiver utilizing a single radio receiver tuned to both channels and a pulse separating circuit actuated thereby.

In the drawings, the radio transmitter and receiver circuits and the tube circuits are shown only in such detail as is necessary to an understanding of the present invention and it is to be understood that the circuits are otherwise of standard and well known form and include the various potential sources and control elements which are well known in the art.

Figure 1 Referring to Fig. l, theblock IB represents a pulse forming circuit the details of which are shown in Fig. 2 and which is adapted to produce a series of spaced. pairs of pulses a and b and to separate the pulses of each pair as indicated in separate circuits terminating at terminals |I .and I2 respectively and having a'common ground terminal I3. The pulse a of each pair of pulses on terminal is assumed to precede the pulse b on terminal I2 although this sequence may be reversed' if desired.

The terminals. I I and |2,.are..connected to poles I4 and I5 respectively of a double pole double throw, reversing switch I6 the stationary contacts of which are connected by lines I1 and I8 respectively, to the input circuits of radio transmitters 23 and 2| respectively. The radio transmittersZUand 2| are preferably of the Shortwave pulse-typeY and are-adapted to radiate carriers on frequenciesV f1' and. f2 amplitude modulated respectively by thepulses a and b. The transmittersmay be. designed to radiate only when a pulse is received from the pulse forming circuit I0, so thatthe: energy radiated constitutes a series of spaced pulses occurring rst on the frequency fr and then on the frequency f2 for switch position R, or vice versa for switch position L.

The pulses radiated by the transmitters 2U and 2| are receivedby the receivers Z2v and 2-3'respec tively which are preferably of the superheterodyne typeterminated with detectors to produce pulses in their output circuits indicated by the terminals 24, 25, 26 and 21. The terminal 24 of the receiver 22 is connected by a line30 to an inputV terminal 3|Y ofatimingicomparator 32, the details. of which arev shownin Fig; 3. The terminal 26 of the receiver 23.is connected by aline 33 to anzinputv terminal 34 of the timing' comparator 32. The terminals 25 and 21 areindicated as connected by a. line 35 to ground and to a common input.Y terminal 3B of the timing comparator 32.

The timing comparator 32.A includesv circuits to be described which areselected so' thatwhen the pulse on channel f1 precedes the'pulse on channel fz, a right relay 39 is energized andwhenA the pulse on the; channel f2 precedes the pulse on the channelv f1 a left relay 38 is energized: These relays arev provided with armatures. 4| and 40 respectively which areadaptedto close work circuits which may constituteselective control for mobile objects suchV as right' and leftA steering controls or a speed control or a suitable signalling circuit, such as an automatic dot-dash receiver.

The arrangement therefore is such that' the relays 39v and 38' will. be selectively actuated in accordancefwith the direction of closure of' the switchA I6 which controls the sequence of the pulses onthe two channels. The switch may, of coures, take the form of a push-button, key, or other suitable device which is readily operated for control or signalling purposes.

The timing comparator circuit 32 is preferably designed so that no operation results unless and until both pulsesV are received and unless the pulses are of sufiicient duration and length and are spaced within the specied time limits. In addition, the system` may be designed to respond only when the pulses on the two channels are repeated with a recurrence rate within specified limits. In this way the system isl made highly selective and free from interference.

Figure' 2 Referring to Fig. 2, the pulse forming circuit is shown as comprising a master oscillator 50 of a type havinghigh stability and designed to oscillate, for example, at 660 cycles per second. The oscillator 50 includes a tube 5I and a frequency control circuit' 52 together with the usual bias and regulating circuits, all of which areof standard construction.

the key tube S5. negative peaks of voltage across secondary 81,

The output circuit of the oscillator 50 includes a phase shitfer 54 comprising a condenser 55 and a resistor 5S connected in series. The resistor 56 is connected in the input circuit of a vacuum tube amplifier 50 including an amplifier tube El and an output transformer 02. The condenser 55 and resistor 50 are preferably of equal numerical impedance at the frequency of operation so that the 'voltage across the resistance 50 leads the input and 66 by a line 60. The plate circuit of the tube `|55 is connected by a condenser 70a and a line l0 `to the output terminal l, and the plate circuit of the tube 66 is connected by a condenser Hd and a line ll to the output terminal I2. The two terminals are connected through resistors to a common grounded return line l2 which is connected to the terminal i3. The tubes 05 and 56 are normally biased by their suppressor grids to an inoperative condition and are designed to be capable of passing current only when a voltage pulse is received from the line 69.

The oscillator 50 is also coupled by line 00 through a phase shifter 8| consisting of a resistance 82 and a condenser 83 to the input circuit of an amplifier tube 84. The phase shifter 8| is similar to the phase shifter 54, but in this f.

case the input circuit of the tube 04 is connected across the condenser of the phase shifter. Hence the voltage supplied to the tube 84 lags 45 behind the voltage from the oscillating circuit. Inasmuch as the voltage applied to the tube 60 leads by a 90 phase difference is produced between the voltages applied to the tubes and 84.

The tube 84 feeds, through an output transformer 85, a pair of rectiers 86a and 85h having cathodes 36o and 86d respectively connected to the two ends of the secondary 8l of the transformer 85 and having anodes connected together `by aline 88 and thence through a resistor S9 and a biasing battery Bil and a return lead 9| to the center tap of the secondary 8l. The biasingbattery 0l] is connected to determine the voltage values at which current Will flow through the `rectiflers due to the voltage impressed by the secondary 0T. Rectifled current ows through a tube internally from plate to cathode, thereafter externally through secondary winding 8l and line 9| to the positive side of Abattery 00, through the battery to the negative side and thence through resistor 80 to the plates 89a and 80h of the rectifying tubes. No current ows in resistor 89 except when the cathode of one of the rectiiiers is negative with respect to line 9| by an amount exceeding the voltage value of battery 90.

The positive side of resistor B0 is connected to the cathode 02 of a key tube 95, and the negative side of resistor 80 is connected to the grid S5 of At both the positive and the the current through resistor B0 is maximum, so

that the grid 26 of tube 95 is then highly negative t with respect to the cathode 92. As the voltage cycle progresses, the grid 96 of tube 05 comes to cathode potential, the rectifying tube cuts off due grid to a negative value. In this manner, the grid ofA tube is actuated by voltage pulses `with a peak value which brings the grid of tube 95 to cathode potential and causes it to remainat that potential a short interval of time to form trapezoidal current pulses occurring at a rate of twice the frequency of oscillator 50. l

The key tube 95 is shown as a pentode, with the cathode 02 positively biased with respect `to ground by a battery |0|, and with the screen 93 and plate -94 positively biased with' respect to the Vcathode by batteries |02, |03. The positive side of battery |03 is connected to the plate of tube 95 through a plate coupling resistor |04. Thesuppresser grid 0l of tube 95 is connected by line 08 to a variable tap 00 on resistor |00, one side of which is connected to the negative and grounded side of battery |0|, and the other side of which is connected to a line |48.

vIn the absence of voltage across resistors 88 and 00, the grid of tube 05 is at cathode potential, but nevertheless no plate current flows because the cathode is highly positive with respect to the suppressor. Therefore the control grid pulses due to rectification of voltage from secondary 87 are not repeated to form corresponding plate current pulses except when the tube 95 is unblocked by a current through resistor |00 which brings the suppressor to the vicinity of cathode potential. A circuit to be described later provides for pulsing resistor |00 recurrently in such a manner that in the embodiment shown the suppressor is brought to the cathode potential for a sufficient time to pass two pulses from the control grid to plate, then suppress the passage of 20 pulses. This is repeated recurrently in such a way that for example two consecutive pulses out of every twenty-two impressed on the grid d actually produce plate current, leaving van interval between centers of pairs of pulses 22 times the interval between centers of thel pulses themselves. f

Due to the use of a high voltage swing from transformer secondary 8l, the key tube 95 also operates as a clipper tube so that only the peaks of the pulses are repeated into the plate circuit, thereby forming rectangular or slightly trapezoidal shaped output pulses.

The plate 9d of tube 95 is connected through condenser H0 to the input grid |09 of a resist- `ance coupled amplifier stage including pentode the output of which is coupled through condenser Htl to line 09. Positive pulses of plate current to tube 95 cause negative pulses of voltage to the grid it?? of tube negative pulses of current to the plate H2 of tube and positive pulses of voltage to the line b9 and therefore to the suppressor grids 5'? and 6B of tubes 65 and 60. The control grids S3, S4 of the tubes 65 and S6, however, are connected in push-pull to the secondary of the transformer 52.- Hence one of these control grids is positive with respect to the center tap of the transformer While the other control grid is negative. When a positive pulse is applied to the suppressor grids 67, 6B from the line S8 a pulse is produced in the output circuit of the tube whose control grid is positive at that instant. The pulses supplied to the suppressor grids are timed by the phase Shifters 5s and 3| to occur on alternate half cycles of the voltage supplied to the control grid. Hence, one pulse from the line |59 is received when the control grid of the tube |55 is positive and the 'next pulse is received when the control grid of the tube 66 is positive. The first pulse is thus passed. through the lead I0 tothe circui-tvincludi118 the output terminal l ,I and the next pulse is passed through the lead 1| to the circuit includving the output terminal |27, this sequence being repeated whenever a pair of pulses is applied from line 69,. These pulses can be considered as positive pulses of current towards the plates of tubes 65 and 66, and as negative voltage pulses from the output terminals to ground.

Pulse control circuit The lead 60 from the oscillator 50 is also connected to an adjustable phase shifter |20 comprising a pair of condensers |2| and a resistor |22 connected in series. The resistor |22 is connected to the input circuit of an amplifier tube |23, the output circuit of which is connected by means of a tapped resistor` |24 to the input circuit of an amplifier |25.

The output circuit of the amplifier |25 is connected through a condenser |26 to one cathode and one anode ||6 of a double diode rectier tube |21, the other anode ||1 of which is connected to a return lead |28 and the other cathode ||8 of which is connected to lead |28 by a condenser |29. The double rectifier tube |21 and the condensers |26 and |29 constitute a portion of a counter circuit which in the embodiment described is designed to operate on a ratio of l1 to 1.

ISuch a counter' circuit is well known in the art being described in RCA Review, July 1940, A precision television synchronizing signal gen erator by Bedford and Smith and is accordingly not described herein in further detail. 660 cycle voltage appearing in the output circuit of the amplifier |25 charges the condenser |26 on one half cycle. When the right hand side |56, ||8 of the double rectifier |21 is conductive the condenser |26 discharges into the condenser |29 and increases the potential of its upper plate. On the next half cycle the right hand side of the double rectifier |21 becomes non-conductive so that the charge remains on the condenser |29, whereas the left hand side ||5, |1 of the double rectifier becomes conductive and allows the charge to leak off of the condenser |26 and bring its right hand plate to ground potential. Hence repeated energies controlled by tube |25 are applied to the condenser |29 until a predetermined voltage is built up therein.

The condenser |29 is connected in the input circuit of a trigger tube |35, the output circuit of which is connected to control the operation of a multi-vibrator |36.

The multi-vibrator |36 is of standard construction and in the form shown includes a pair of tubes |31 and |38 having screen and control grids which are cross connected, the respective grids being connected to a resistance network including a pair of resistors |40 having variable taps |4|. The arrangement is such that the normal frequency of operation of the multi-vibrator |36 in the absence of synchronization will be oneeleventh of 660 cycles, that is 60 cycles. This free running value will be controlled to a limited degree by the setting of the taps |4| on the resistors |40.

The trigger tube |35 is connected to be pulsed when a predetermined voltage is applied to the input circuit thereof by the condenser |29. The output circuit of the trigger tube is connected to the oscillating circuit of the multi-vibrator |36 in such a way that the multi-vibrator falls in step with the pulses of the trigger tube over a substantial variation in setting of the taps |4|.

In the embodiment .disclosed the cathode. battery |35a of tube |35 and thecondenser |20 are so chosen .so that the condenser will 'be charged to a sufficient voltage to cause the trigger tube 35 to operate and pass current when eleven charges are received from the condenser |26. By this triggering operation, a synchronizing control voltage is impressed upon the screen |313- of tube |31, due to the current through tube |35. 'I'his control voltage also causes the plate to cathode branch of tube |31 to be highly conductive and discharges the condenser to or nearly to ground potential. Thereupon eleven more cycles of charge on condenser |29 again cause the trigger operation. Hence the multi-vibrator |36 is caused to operate accurately on a frequency one eleventh of that of oscillator 50, that is at 60 cycles per second. Slight adjustments of phase of the multi-vibrator oscillations can be made by adjusting the capacitance of the phase shifting condenser |2|y and by adjustment of the double resistor |40.

The multi-vibrator is designed to produce a 60 cycle alternating pulse in the plate circuit of tube |38 with spaced peaks as distinguished from a sine wave. These pulses are applied across a resistance |42 in the input circuit of a clipper tube |43 which is biased by a battery |43a to operate only at the peaks of the input pulses. The plate circuit of the clipper tube |43 includes a plate resistance |44 which lowers the plate voltage when plate current flows therein.

The plate |43b of the clipper tube |43 is connected to the input circuit of an amplifier tube |45 which is normally biased to pass current and is provided with a plate resistance |46. The plate current of this tube |45, however, is reduced when the grid potential is reduced due to a reduction in plate voltage of the tube |43. This reduction in plate current through the plate resistance |46 causes an increase in the plate voltage in the form of a positive voltage pulse which is supplied by a condenser |41 and lead |48 to the resistor |00 above mentioned.

The operation of the multi-vibrator |36 and the bias of the clipper tube |43 are such that the output voltage pulses in the tube |45 are of a duration coresponding to a pair of pulses on the control grid 96 of the tube 95 and are so timed by phase adjustment that the positive bias thus supplied to the suppressor grid 91 of the tube 95 serves to produce a single pair of pulses in the output circuit of the tube 95. These pulses are supplied to the suppressor grids 61, 68 of the tubes 65 and 66 as above described. With the multi-vibrator |36 operating at 60 cycles, a single pair of pulses is released each 60th of a second. Since the pulses themselvesare derived from a 660 cycle wave and are separated by one half a cycle, the spacing of the centers of successive pairs of pulses is twentytwo times the spacing of the individual pulses of each pair.

Operation of Figure 2 The operation of the pulse forming circuit will be better understood by referring to the series of curves A to G shown in Fig. 2A. As previously pointed out, a 60 cycle pulse of exactly one eleventh the frequency of oscillator 50 is produced by the multi-vibrator |36. The peaks of this pulse are clipped by the clipped tube |43 and are applied to the amplifier tube |45 to produce a series of spaced rectangular pulses, one of which is represented by the curve G in Fig. 2A. 'I'hese pulses are supplied to thesuppressor grid 91 of the tube '95 and serve'to unblock the tube 95 during the pulse intervals.

A pulse is applied to the control grid 96 of the tube 95 each time the voltage in the secondary 81 of the transformer 35 passes through zero. The secondary voltage is indicated by the curve E of Fig 2A and the pulses applied to the control grid 96 and repeated as plate current are represented at a and b on the curve F. These pulses a and b are continuously repeated on the control grid 96 but only appear in the output circuit of the tube 95 when they coincide with a pulse on the suppressor grid. Hence, only pairs of pulses are produced in the output circuit of the tube 95 as indicated by the curve F.

The voltages supplied to the control grids 6 3, E4 of the respective tubes 65 and 65 from the master oscillator 58 are represented by the curves C and D of Fig. 2A. It is to be noted that these voltages are both of 660 cycles frequency but are 180 out of phase due to the push-pull connection of the two control grids. When the pulses a and b from the output circuit or" the tube iii are received on the suppressor grids S1, 63 of the tubes 65 and 55, one or the other of the tubes S5 or 56 is rendered conductive, depending upon which of the tubes is receiving a positive bias on its control grid at that instant.

As illustrated in Fig. 2A, the curve E is displaced 90 with respect to the curves C and D pair will appear in the output plate circuit of theV tube 65 as indicated by the curve A of Fig. 2A and the second pulse of each pair will appear in the output plate circuit of the tube 55. These pulses cause corresponding negative voltage pulses to occur at output terminals ii and i2. The phase shifter |2|l and resistors I4@ are adjusted to obtain the proper phase relationship between the curve G and the curve F for causing pairs of pulses to be produced.

Referring now to Fig. l, the pulses in the output circuit of the pulse forming circuit are shown as connected through control switch It to modulate transmitters 2B and 2| which, as previously pointed out, are operating on different channels. The pulse a at the terminal always occurs before the pulse b at the terminals I2. The control switch It determines to which of the transmitters or 2| the first pulse is to be applied. Hence the output waves of the transmitters representa pulse on the channel f1 followed by a pulse on the channel f2 or vice versa, depending upon the position of the switch I6.

Receiving apparatus The waves radiated from the transmitters 29 and 2| of Fig. 1 are shown as received by the receivers 22 and 23 which are tuned to the respective wave frequencies and are of standard construction to make the received pulses available in the output circuits of a detector. The detectors are of a type which will supply positive pulses at terminals 24 and 26 with respect to ground terminals 25 and 2l. These pulses are supplied to the circuits including input terminals 3| and 34 of the timing comparator 32. The pulse at the terminal 3| either precedes or lags behind the pulse at the terminal 34 depending l0 upon the sequence of the transmitted pulses.- The circuit for the timing comparator 32 is shown more in detail in Fig. 3.

Figure 3 Referring now to Fig. 3, the terminal 3| is connected by a line It!) to a resistor iti in the input circuit or" an amplifying triode it`2. The terminal 361 is connected by a line |83 to a resistor its in the input circuit of an amplifying triode H35. The return leads from the resistors ll and itil are connected to the terminal 35. The plates S, Sti of the triodes |62 and |55 are fed through resistors E65 and 25'! respectively from a line itt connected to a source of plate potential shownas a battery H9. For simplicity of construction, the triodes |82 and |55 may be housed in a common envelope,r as may other triodes of the Fig. 3. They will be described however for convenience as independent tubes.

The pulse output of the tube it? is fed through a condenser lli to the control grid 302 of a tube H2. The plate SS of tube E?? is connected through a condenser H3 to the control grid 304 of a tube illi. The cathodes of the tubes il? and lill are connected to ground through a biasing battery are. The plates its of the tubes liz and i'il are connected tothe line |68 through resistors titi and i'ii respectively. rlihe plate 3&35 of the tube iid is also connected through a resistor iili with the control grid 32 ci the tube il? and through a resistor il@ and a condenser 589 to the control grid tilt of a key tube i8l. The output circuit of the ampiier tube It is connected by a line it?. through a resistor 83 to the control grid 39'! of an ampliiier tube |84. The plates Ebd, oi the tubes ll' and |34 are connected in parallel through a resistance E35 to the line iba.

The tubes H2 and illi and associated circuits constitute in eiect a pulse stretching system Which operates when energized by a short negative pulse on the input grid terminal of tube H2 to produce a negative pulse of longer duration and in the same sense on the output circuit of companion tube V54. The output of one tube shown as tube H2 is capacity coupled to the input of the other tube H4 as in a multi-vibrator, but the output of the tube lid is directly coupled to the input of the tube H2 as in an electrical toggle. This system has one stable equilibrium position in the absence of signals. 'When `the condition of stable equilibrium is momentarily upset, as by a pulse on grid 3212 of tube H2 from the plate SB@ of tube |52, the system does not instantaneously return to the equilibrium condition, because of the energy change in condenser ITS during the pulsing. As a result the plate currentv of output tube iis continues to change after the pulse on tube H2 has passed.

In the embodiment shown in the equiiibriuni condition tube lili is biased to below cutoii' by battery H5, While tube |12 passes eurent due togrid 302 of tube H2. In the equilibrium condition with no current flowing to or from the con-f denser |73 its lower plate is at ground potential and its upper plate is at the potential of the line |58 diminished byv the very heavy drop, through resistor H6 to the plate S03 of thecurf-` rent carrying tube |72. When now the negative.;

pulse is impressed upon the control grid 302 of tube |12, the plate current is momentarily diminished toward or to zero, thereby decreasing the voltage drop in resistor |16 and increasing the potential on the upper plate of condenser |13. This tends to make the upper plate of condenser |13y more positive by a charging current to the condenser from the plate 303 of tube |12, and this charging current, represented in the condenser as a displacement current, causes corresponding current to flow from the lower plate of condenser |13 to ground through the grid resistor 3|0. As a result, both the upper plate and lower plate of the condenser |13 are raised to a higher potential, and the grid 304 of tube |14 which is connected to the lower plate of condenser |13 is quickly raised to the potential of the cathode of tube |14 or higher. This in turn increases the current flow to the plate 305 of tube |14, thereby lowering its plate potential due to the increased drop through resistor |11. Due to the direct coupling from plate of tube |14 to ground through resistor |18, the lowering of plate voltage of tube |12 drives its grid negatively in the same sense as the original pulse. 1f condenser |13 were of infinite capacity, the grid 302 of tube |12 would continue to be held negative, so that the controlling pulse from tube |62 would be followed by a permanent holding pulse from tube |14. However due to the capacity of condenser |13 being finite, the potential across condenser |13 changes in accordance with the voltage and resistance in its external circuit. With the grid of tube |12 driven negatively beyond cutoff, making-tube |12 currentless; condenser |13 is charged to increase its voltage toward the limiting value of battery |10. But as it approaches this value, the charging current diminishes, decreasing the current through the grid-to-cathode and grid-toground path for tube |14, so thatr tube |14 conimences to draw less current. As a result the potential of tube |12, until now held below cutoi after the control pulse from tube |62 has passed', approaches the cutoff point. When during the charging of condenser through resistor |16, the cutoff point of tube |12 is reached, and it commences to pass plate current also through the resistor |16, the charging oi' condenser |13 is checked due to the lowering of voltage applied to the top plate of condenser |13, the grid voltage of tube |12 drops toward zero and due to the coupling from tube |14 to tube |12 the action of causing tube |12 to pass plate current is accelerated. With the charging of' condenser |13 checked, the operation of the tubes isl such as to cause the condenser to discharge to its equilibrium condition. During this discharge operation, the voltage on the grid 304 of tube |14 is highly negative beyond cutoff, so that equilibrium is reached with no further change in plate current of tube |14.

By this action, the negative pulse impressed for a short time by the tube |62 upon the grid of tube |12 causes an extended positive pulse to appear on the grid of tube |14, and an extended negative pulseto be impressed from the plate of tube |;14 onto the grid 306 of tube |8|.

The alternating current output of the amplifying tubev |65 is transmitted throughV line |82 and resistor |83 to the control grid 301 of triodetube |84; Consequently through the action of tubes |62, |12 and |14, a positive pulse applied to terminal 3| causes a negative pulse with the same starting time, but of longer duration to be impressed upon the input of tube |8|. Also by |13 by a current the action of tube |65, a positive pulse applied to terminal 34 causes a corresponding unstretched negative pulse to be impressed upon the input of tube |84. By suitable choice of the resistors and the condenser |13 associated with pulse stretching tubes |12 and |14, the pulse on tube |8| may be caused to last longer than the time interval between pulses on terminals 3| and 34. Therefore in the event that terminal 3| is pulsed first and terminal 34 later, within a predetermined time limit established by the action of tubes |12 and |14, overlapping pulses will be impressed on the two tubes |8| and |84. If however terrninal 34 is pulsed rst then the pulse on tube |84 will have passed before the pulse on tube |8| starts, and there will be no overlap of these pulses.

The tubes |8| and |84 are coincidental keyl tubes, and in the embodiment shown they are triodes, with the cathodes connected together and grounded by a line 3|| and with the plates 308, 309 connected together and fed from the line |68 through the resistor |85. The plates 308. 309 are direct coupled through resistor to the grid 3|2 of a clipper tube |9| having its cathode biased positive with respect to ground by a battery 3|3 and its plate 3|4 fed from the line |69 through a resistor |93. The grid 3 I2 of tube |9| is positively biased by the direct current flowing through resistors |85 and |90 and through the grid resistor 3|5 to ground, but the cathode battery 3 I3 biases the cathode of tube |9| to a much higher potential than its grid, so that as a net result, tube |9| is biased considerably beyond cutoff so that normally no plate current flows through resistor |93.

Circuit conditions are so adjusted that no current passes through resistor |93 until the grids of both tubes |8| and |84 are very considerably negative. In the absence of a pulse on tube |8| for example, cutoff of tube |84 by a negative pulse will cause a change of plate current through resistor |85, but no effect in resistor |93. Only when the pulses on tubes |8| and |84 are coincidental will there be a pulse through resistor |93.

It is possible to utilize the pulse through resistor |93 to trigger off a gaseous relay tube, but in the present embodiment, use is made of the fact that the pulses established in resistor |93 are of a recurrent nature.

The plate 3|4 of clipper tube |9| is connected through condenser |94 to the grid 3|6 of amplifying triode |95, which in turn is connected to ground by a resistor 3|1 and condenser 3|8` in parallel. The cathode of the triode ampliiier |95 is grounded by line 3|9 and its plate 320 is connected to line |68 through a plate resistor 32|.

The plate 320 of tube |95 is coupled through a condenser 322 to the anode 323 of a rectifier 200, the cathode of which is connected to ground through resistor 20|, bypassed by a condenser 324, and also is connected to the grid 325 of a relay tube 202. A resistor 326 connected from the anode 323 of rectifier 200 to ground provides a D. C. return path for rectifier 200. The plate 321 of relay tube 202 is connected to the winding 39 of a relay 328, the other side of which is connected to line |68, and the cathode of relay tube 202 is positively biased by a battery 329 so;

that no plate current will flow unless there is current passing through resistor 20| due to action of the rectifier 200. The relay 328 is provided with an armature 4| for closing'an external work circuit.

The constants of the circuit4 associated with amplifier |95 are so chosen that it selectively integrates and ,amplifiesthe pulse power derived from the plate of clippertube |9|. This produces a Wave form with high proportion ofienergy content in the fundamental rate of pulsing which is impressed on rectifier G. This wave form is rectified, and smoothed out by the cathode condenser 324 of tube 200, and the D. C. component of the rectied current raises the grid of relay tube 202 to cathode potential causing plate current to iiow and attract the relay armature d In the operation of Fig. 3 thus far discussed, when upon closure for example of switch it tc position R a positive pulse is impressed upon terminal 3| with respect to ground terminal 33, followed by a positive pulse upon terminal 32 within a predetermined time limit, amplifier |32 of Fig. 3 impresses a negative pulse upon pulse stretching circuit involving tubes |12, |14 and resulting in a negative pulse of longer duration impressed upon the grid 306 of coincidental key tube |8|. The duration of this pulse overlaps the time interval at which a negative pulse is impressed upon coincidental key tube |84 due to the later pulse received from tube |65. By the coincidental action of tubes |8| and |84, clipper tube I9| is pulsed positively so that a pulse flows in the plate resistor |93 during the interval of overlap. Thispulse is broadened, integrated and amplied by tube and rectified by tube 2l to cause operation of the relay armature 4|.

In the alternative event that switch iS of Fig. 1 is thrown for exampleV to position L the pulse on terminal 34 precedes that on terminal 3| the circuit above described will operate to produce pulses in resistor |35 whichv are insufficient to cause operation of the clipper tube |95. For utilizing the possible control corresponding to terminal 31| being pulsed before terminal 3|, a companion circuit 229 is provided. This may be of the same general construction as that previously described, with corresponding parts designated by `like numbers but followed by the kletter a. However terminal 34 is vrconnected to `drive the grid of tube |6211 and terminal 3| is connected to drive the grid of tube |a. Therefore the pulse delay circuit 232 responds to a pulse on terminal 3d. The output of circuit 29 includes a relay 32M with armature fill.

In this manner, the circuit of the timing ccmparator of Fig. 3 provides for operation of left relay 32811 when the switch i3 is closed to position L which pulses channel f2 before fi and it provides for closure of right relay 328 when the switch il is closed to position R which pulses channel f1 before f2.

Figure 4 In the system shown in Fig. 1 two independent receivers 22 and 23 are provided which are tuned respectively to the channels f1 and f2. Fig. 4 shows a circuit for receiving both channels on a single intermediate frequency type receiver and separating the pulses for application to the timing comparator. Referring to Fig. 4, the block 2li) indicates a receiving circuit which is tuned broadly to receive the two channels f1 and f2. A single frequency local oscillator 2|| and a detector 2|2 are connected to make the two pulses available as intermediate frequency pulses in an output circuit transformer 2|5 tuned to both intermediate frequencies in a well known manner. These pulses are supplied through the transformer 215 to an amplifier tube 2|5, the output circuit of which is connected to a well known type of frequency discriminating circuit `2|7 which includes coupled inductors 2|8 and 2|9 forming parts of a coupled circuit system and connected at their mid-points through a condenser 220. The mid-'points of the inductors 2|8 and 2|9 are also connected through a' resistor' 222 and an inductor 223 to the return lead 224 of the amplifier tube 2|6. I

The frequency discriminating circuit 2|1'has characteristics such that one of the pulses, for example, the intermediate frequency pulse correspending to that received on the frequency f1, may be derived from one end of the inductor 2|9 and applied by a lead 225 to an amplifier tubel 226, whereas the intermediate frequency pulse corresponding to that received on the frequency f2 may be-derived from the other end of the inductor 2|9 and applied by a lead 221 `to an ampliiier tube 223. The output circuits of the amplifier tubes 226 and 222 are connected through selective intermediate frequency transformers 230 and f 23| respectively to rectiiiers V232 and 233 respectively. Resistors 234 and 235, in circuits with the rectiers 232 and 233, are connected respectively across terminals 24 and 25 and across terminals 26 and 2l which correspond to the terminals 24, 25, 26 and 2l of Fig; 1. The pulses are thus separated and made individually available as positive pulses to the timing comparator.

The operation of this embodiment is similar to that above described except that only a single receiver is used instead of the tworeceivers indicated in Fig. 1. i

In the embodiment of Fig. 4 a pair of ypulses on channels f1 and fz are received and detected in the tuner 2 i@ and detector 2|2 andare applied to the amplifier 2|3 through a double peaked transformer 2|5 with transmission peaks corresponding in spacing to the two frequencies f1 and f2. These pulses are separated by the frequency discriminator circuit 2|? and are individually amplified in the amplifiers226 and 228. The amplifier outputs are rectified by the rectiiiers 232 and 233 to produce voltage drops in the resistors 232 and 235 in the form of voltage pulses corresponding to the received pulses. The pulse received on the frequency f1 is thus supplied to the terminals 24 and 25 and the pulse received on the frequency f2 is supplied to the terminals 26 and 2l. These terminals are connected to the timing comparator 32 as shown in Fig. 1 wherein their timing is compared and the relays 328 and 323e are selectively actuated in accordance with the pulse sequence.

It is to be understood that a plurality of channels may bel usedwhich may be pulsed in selected sequences for a multiple control. A pair of channels have been described for purposes of illustration only.

I claim as my invention:

1. A radio signalling system comprising radio transmitting means to propagate radiant energy on a plurality of radio frequency channels, a puise forming circuit to form a series of energy pulses having a predetermined time sequence, means modulating said radio transmitting means with said pulses to selectively propagate the individua1 pulses of said series on different radio frequency channels, means controlling the relative sequence in frequency of said pulses on the various channels for signalling, radio receiving means responsive to said radiated pulses including means selective of the pulses on the various channels, and a circuit selective of the pulse sequenceon the' various channels connected tov respond only to a predetermined relative sequence and only two said pulses being within apredetermined time interval of each other.

2. A radio signalling system comprising radio transmitting means to propagate radiant energy on a pair of radio frequency channels, a pulse forming circuit to form a pair of pulses in predetermined time sequence, means modulatingv said transmitting means with said pulses to propagate one pulse over one channel and the other pulse over the other channel in sequence, means controlling the relative sequence in frequency for signalling, radio receiving means to receive and separate said pulses. and means responsiveto a predetermined relative pulse sequence and only with the second pulse thereof within a predetermined time interval of the first pulse of each pair.

3. A radio signallingv system comprising radio transmitting means to propogate radiant energy on a pair of radio frequency channels, a pulse formingv circuit to form a pair of pulses in predetermined time sequence, means modulating said transmitting means with said pulses to propagate one pulse over one channel and the other pulse over the other channel in sequence, means to select the relative sequence in the frequency channel for signalling, radio receiving means to receive and separate said pulses, a circuit selectively responsive to one relative pulse sequence and a second circuit responsive to the reverse pulse sequence, said circuits being thus responsive only with the second pulse of each pairl within a predetermined time interval of the first pulse thereof.

4. A radio signalling system comprising radio transmitting means to propagate radiant energy on a pair of radio frequency channels, a pulse forming circuit to form a pair of pulses in predetermined time sequence, means modulating said transmitting means with said pulses to propogate one pulse over one channel and the other pulseover the other channel in sequence,

means to select the relative sequencey in the frequency channel for signalling, radio receiving' means to receive and separate said pulses, a pair of channels individually selective of the relative pulse sequences, said channels being operable only with the second pulse of said pair of pulses within a predetermined time interval after the first pulse of said pair, and relay means actuated by each channel.

5. Thev signalling system claimed in claim 44 each of said channels comprising pulse stretching means to stretch the first pulse of each pair for said predetermined time interval andv further comprising a coincidence circuit responsive only to the coincidence of a portion of saidA first pulse with said second pulse.

6. The signalling system' claimed in claim 5, said pulsev stretching circuit including a multivibrator' circuit. Y

ELLISON S.. PURINGTON.

The following references are of record in the:

iii'e of this patent:

UNITED STATES PATENTS Number Name Date 1,724,227 Starr Aug. 13. 1929 1,743,588 Afel et al. Jan. 14, 1930 1,801,146 Finch Apr.v 14,1931 1,889,292 Ricchiardi Nov. 29, 1932 2,001,747 Runge May 21, 1935 2,070,418 Beverage Feb. 9, 1937 2,188,147 Moore et al Dec. 12, 1939 2,325,829 Boswau Aug. 3, 1943 2,395,478 Hansell Feb. 26, 1946 2,412,974 Deloraine Dec. 24, 1946 2,414,103 Hunter Jan. 14,1947 2,453,659 Bellescize Nov. 9,. 1948 2,464,667 Boosman et al Mar. 15, 1949 FOREIGN PATENTS Number Country Y Date 348,937 Great Britain May 21, 1931 

